This invention pertains to an implantable medical device, and more particularly, to an implantable cardioverter-defibrillator (ICD), which senses a dangerous cardiac arrhythmia and, in response, provides therapy to a patient""s heart to revert it to a normal sinus rhythm. More particularly, this invention pertains to an ICD in which a depolarization pulse is applied after a defibrillation shock, and/or the electrodes used to deliver the defibrillation shock are shorted together for a brief time period, to discharge any residual charges, thereby insuring that intrinsic cardiac signals indicative of fibrillation are not masked.
As used herein, the term xe2x80x9carrhythmiaxe2x80x9d refers to any abnormal heart rhythm that may be dangerous to the patient and specifically includes fibrillation, atrial tachycardias, supraventricular tachycardias (SVT), ventricular tachycardias (VT), ventricular fibrillation and flutter (VF). As further used-herein, the term xe2x80x9ctherapyxe2x80x9d refers to any means used by the ICD device to restore normal heart rhythm, such as defibrillation, cardioversion, and antitachycardia pacing. The term xe2x80x9ccardioverterxe2x80x9d refers to a device capable of providing defibrillation therapy, cardioversion therapy, or both.
Typically, defibrillation therapy consists of the application to cardiac tissue of one or more electrical shocks of considerable amplitude and duration. In cases where a first defibrillation shock is not successful, a second shock having much smaller amplitude applied within about 1-2 seconds after the first shock may suffice to revert the heart to normal sinus rhythm. It is desirable to apply a subsequent defibrillation shock as soon as it is discovered that the heart has not reverted despite earlier attempts.
However, such therapy immediately after the delivery of a defibrillation shock has not always been possible because it may not be possible to sense the on-going arrhythmia (including VF) for many seconds after a defibrillation shock is applied. More particularly, until now such early therapy additional (e.g., second-shock) could not be applied because the first defibrillation shock results in a build-up of residual charge on the electrodes and a local polarization of the tissues which would dissipate only after about 10 seconds. This may mask any low amplitude VF, and necessarily lead to a delay in the application of another shock.
In its broadest sense, the present invention pertains to an ICD wherein a two-step process is executed after the administration of high energy level therapy such as a defibrillation shock consisting of one or more high amplitude pulses. The process consists of first applying a relatively short duration and low amplitude pulse, preferably having a polarity opposite to that of the last pulse of the therapy, to depolarize the electrodes thereby dissipating any residual or parasitic charges therein. Following this short duration pulse, the electrodes used to apply the therapy are momentarily shorted together to discharge any remaining residual charges.
In a particularly advantageous arrangement, a multiphasic shock is generated by a circuit that includes a capacitor charged to a preselected voltage of at least 100V and then discharged through a set of electronic switches arranged in a bridge-like fashion. In this arrangement, the short duration pulse is generated by applying a charge from the capacitor to the electrodes using the same switches that control or steer the therapy shock. After the short duration pulse, additional switches are used to short the electrodes together.
Advantageously, the electrodes may also be shorted to the conductive case of the ICD, especially in arrangements where the conductive case acts as anelectrode as well.